The read-back sensor in a hard disc drive recording head retrieves the data stored on a magnetic storage disc. Magnetoresistive (MR) thin film sensors have become the industry-standard technology for this critical role, with the technology evolving over the past decade from anisotropic magnetoresistance (AMR) to giant magnetoresistance (GMR) to, most recently, tunneling magnetoresistance (TMR). This evolution in reader technology will likely continue as the hard drive industry strives for further increases in areal density and data rate.
Readers will have to become physically smaller and operate at higher recording frequencies that are expected to be on the order of a few GHz. This scaling trend poses two significant problems for TMR sensors. First, the resistance of the sensor will inevitably increase as the sensor size (or, more specifically, the junction area) decreases, leading to more noise without the prospect of more amplitude. Secondly, ferromagnetic-based sensors, such as TMR sensors, have to contend with thermally activated magnetic noise that limits performance to frequencies below a resonant frequency that is typically on the order of a few GHz.
TMR-based readers are expected to work up to an areal density of approximately 500 Gbit/in2. Alternative magnetoresistive technology, such as current-perpendicular-to-the-plane (CPP) GMR sensors, current-confined-path (CCP) GMR sensors, and extraordinary Hall effect (EHE) sensors, may achieve the combined amplitude and noise requirements at higher areal density. However, like TMR sensors, these new technologies will still have to contend with the noise and bandwidth limitations imposed by thermally activated magnetization dynamics. Hence, ferromagnetic-based MR technology will likely have only limited extendibility to higher recording frequencies. While clever engineering may be able to prolong the use of MR technology to densities beyond 500 Gbit/in2, readers that rely upon magnetoresistance and simple magnetization rotation to transduce media flux into a read-back voltage will ultimately become inadequate if current industry trends continue.
Thus, there is a need for a reader technology that overcomes the limitations of prior art devices.